Observer Pattern
With the observer pattern, we can subscribe certain objects, the observers, to another object, called the observable. Whenever an event occurs, the observable notifies all its observers!
When to Use
- Use this when you need to notify multiple parts of an application about state changes or events
- This is helpful for implementing event-driven, asynchronous communication between components
Instructions
- Create an Observable class with , , and methods
- Keep observers loosely coupled to the observable for better separation of concerns
- Be mindful of performance when notifying many subscribers with complex logic
- Consider using libraries like RxJS for more advanced reactive programming needs
Details
An observable object usually contains 3 important parts:
- : an array of observers that will get notified whenever a specific event occurs
- : a method in order to add observers to the observers list
- : a method in order to remove observers from the observers list
- : a method to notify all observers whenever a specific event occurs
Let's create an observable using an ES6 class:
js
class Observable {
constructor() {
this.observers = [];
}
subscribe(func) {
this.observers.push(func);
}
unsubscribe(func) {
this.observers = this.observers.filter((observer) => observer !== func);
}
notify(data) {
this.observers.forEach((observer) => observer(data));
}
}
We can now add observers to the list of observers with the subscribe method, remove the observers with the unsubscribe method, and notify all subscribers with the notify method.
Let's build something with this observable. We have a very basic app that only consists of two components: a
, and a
.
js
export default function App() {
return (
<div className="App">
<Button>Click me!</Button>
<FormControlLabel control={<Switch />} />
</div>
);
}
We want to keep track of the user interaction with the application. Whenever a user either clicks the button or toggles the switch, we want to log this event with the timestamp. Besides logging it, we also want to create a toast notification that shows up whenever an event occurs!
Whenever the user invokes the
or
function, the functions invoke the
method on the observer. The
method notifies all subscribers with the data that was passed by the
or
function!
First, let's create the
and
functions. These functions will eventually receive
from the
method.
js
import { ToastContainer, toast } from "react-toastify";
function logger(data) {
console.log(`${Date.now()} ${data}`);
}
function toastify(data) {
toast(data);
}
export default function App() {
return (
<div className="App">
<Button>Click me!</Button>
<FormControlLabel control={<Switch />} />
<ToastContainer />
</div>
);
}
Currently, the
and
functions are unaware of observable: the observable can't notify them yet! In order to make them observers, we'd have to
subscribe them, using the
method on the observable!
js
import { ToastContainer, toast } from "react-toastify";
function logger(data) {
console.log(`${Date.now()} ${data}`);
}
function toastify(data) {
toast(data);
}
observable.subscribe(logger);
observable.subscribe(toastify);
export default function App() {
return (
<div className="App">
<Button>Click me!</Button>
<FormControlLabel control={<Switch />} />
<ToastContainer />
</div>
);
}
Whenever an event occurs, the
and
functions will get notified. Now we just need to implement the functions that actually notify the observable: the
and
functions! These functions should invoke the
method on the observable, and pass the data that the observers should receive.
js
import { ToastContainer, toast } from "react-toastify";
function logger(data) {
console.log(`${Date.now()} ${data}`);
}
function toastify(data) {
toast(data);
}
observable.subscribe(logger);
observable.subscribe(toastify);
export default function App() {
function handleClick() {
observable.notify("User clicked button!");
}
function handleToggle() {
observable.notify("User toggled switch!");
}
return (
<div className="App">
<Button>Click me!</Button>
<FormControlLabel control={<Switch />} />
<ToastContainer />
</div>
);
}
We just finished the entire flow:
and
invoke the
method on the observer with the data, after which the observer notifies the subscribers: the
and
functions in this case.
Whenever a user interacts with either of the components, both the
and the
functions will get notified with the data that we passed to the
method!
Although we can use the observer pattern in many ways, it can be very useful when working with asynchronous, event-based data. Maybe you want certain components to get notified whenever certain data has finished downloading, or whenever users sent new messages to a message board and all other members should get notified.
Case study
A popular library that uses the observable pattern is RxJS.
ReactiveX combines the Observer pattern with the Iterator pattern and functional programming with collections to fill the need for an ideal way of managing sequences of events. - RxJS
RxJS has tons of built-in features and examples that work with the observable pattern.
Pros
Using the observer pattern is a great way to enforce separation of concerns and the single-responsibility principle. The observer objects aren't tightly coupled to the observable object, and can be (de)coupled at any time. The observable object is responsible for monitoring the events, while the observers simply handle the received data.
Cons
If an observer becomes too complex, it may cause performance issues when notifying all subscribers.
Source
References